This invention relates to a highly efficient engine employing an unsymmetrical expansion and compression cycle along with the use of a supercritical mixture of fuel and water.
The operation of internal combustion engines is generally a trade-off between efficiency of the engine and cleanliness of the exhaust. For example, diesel engines provide high efficiency but the exhaust usually includes particulate matter (PM) such as soot and nitric oxide (NO.sub.x). In general, internal combustion engines, whether spark ignition or diesel, operate on a symmetrical cycle. That is, compression volume equals expansion volume. Ralph Miller in U.S. Pat. No. 2,670,595 was the first to describe an engine operating on an unsymmetrical cycle. He recognized that closing intake valves either before or after bottom dead center (BDC) could change the "effective" compression ratio in an engine.
For example, by doubling the length of the stroke of the engine and closing the intake valve early when the piston is halfway down toward bottom dead center (BDC), the amount of compressed air is reduced by one-half, giving the same effective compression ratio as the original engine. No work is done on the air inside the cylinder during expansion to bottom dead center and subsequent compression back to atmospheric pressure. At the same fuel-air ratio the same peak pressures will be felt by the engine components. If the expansion ratio is left unchanged, then the combusting gases can expand to twice the volume. This increased expansion reduces heat loss to the exhaust and allows the combustion products to do additional work by reaching a lower temperature prior to opening of the exhaust valve(s). Thereby, the engine extracts more useful mechanical energy.
Historically, this unsymmetrical cycle innovation went largely unnoticed because it called for an increase in size and weight for the engine providing the same power. This size and weight penalty was unacceptable to the engine industry of the 1950's and 1960's during which time the single most important figure of merit was horsepower/cubic inch of engine displacement. Also, Miller's proposed mechanism for variable valve timing was cumbersome and did not provide as much valve timing angle variation as desired.
In 1992 Ozawa (see U.S. Pat. No. 5,682,854) discussed ways of overcoming this deficiency in power output by developing a variable compression/expansion ratio. His system used a planetary gear drive designed to alter the position of the intake valve camshaft. Mechanical actuators on the planet carrier physically rotated the camshaft forward or backward in response to the engine's need for power at the expense of engine cycle efficiency. Ozawa's innovation has been commercially realized in vehicles built by the Mazda Corporation. In particular, the Mazda Millennia, introduced in 1994, employs a continuously variable cam component to combine high power capability with high efficiency.
Engines generally use solid metal pistons and cylinder heads and because of the solid metal, the a thermal diffusivity is high. The thermal diffusivity is selected so that the surface temperature of the pistons and cylinder heads remain low enough to avoid thermal stress cracking of the surface under the repeated cyclic heating resulting from engine combustion. This avoidance of thermal stress cracking requires relatively low operating temperatures (300.degree. F.-500.degree. F.) for aluminum pistons and heads. This temperature is maintained by the engine cooling system removing the heat transferred by the combustion process. Of course, such heat is not available for conversion to work in the engine expansion cycle. Thus, heat transfer through solid metal pistons and heads results in loss of efficiency. A reduction in such heat transfer through pistons and cylinder heads will therefore improve the overall thermal efficiency of the engine.
Copending U.S. patent application Ser. No. 08/992,983 filed on Dec. 18, 1997 discloses a supercritical water/fuel composition and combustion system in which a mixture of water and a hydrocarbon fuel is maintained near or above the thermodynamic critical point such that the mixture is a homogeneous single phase. As taught in that application, because the water/hydrocarbon fuel mixture is maintained as a homogeneous isotropic single phase it will combust more completely when introduced into a combustion chamber.
It is well known in the engine system art that liquid fuel combustion relies on spray atomization followed by fuel droplet evaporation and finally on the combustion reaction sequence. Smaller droplets favor more complete and cleaner combustion. Prior art approaches used extremely high injection pressures to minimize the droplet diameters. Fuel preheating and chemical surfactants produce smaller droplets, but such fuel preheating and chemical surfactants produce only modest reductions in droplet size and heating is only effective up to 150.degree. C. to 200.degree. C. Above these temperatures excessive coke, gum, and tar formation blocks operating flow channels in the fuel delivery system.
It is therefore desirable to create a highly efficient engine system by combining an unsymmetrical cycle with a supercritical water/fuel mixture along with controlled thermal diffusivity in the piston and cylinder head.